The image carrier in rotogravure printing is the cylindrical copper outer surface of a printing roll or gravure cylinder. The image carrier is engraved or etched with an image made up of small depressions, called cells, which hold ink. The image and its carrier are protected against the abrasion of the doctor blade in the printing press by a plated chrome layer. The ink selectively retained by the cells of the image is transferred onto a web of paper or another substrate in a rotogravure printing press.
The base of the gravure cylinder is the physical structure which receives and supports the image carrier. Most cylinder bases are made completely of steel. Steel can be machined accurately and plated with other metals rather easily.
Copper is the dominant image carrying surface material for gravure cylinders. Copper is applied to the steel cylinder in three steps. First, an initial flash (an adhesive layer only a few microns thick) of copper is plated to the steel with a cyanide electrolyte. As an alternative, the steel base can be plated with a nickel layer. Nickel plating tanks need more attention to achieve good results. Next, an underlying "base copper" layer 0.5 mm to 1.0 mm thick is electroplated onto the base with a sulfuric acid based electrolyte. Finally, the engraving surface, which serves as the image carrier, is electroplated on the base copper, again using a sulfuric acid based electrolyte.
An exemplary base copper layer has a diameter 160 to 200 microns (0.0063 to 0.008 inches) below printing diameter. The base copper layer should have as good a finish as the subsequently applied engraving layer. The base copper can be prepared either with a lathe and grinder or a machine tool.
The type of image carrier particularly relevant here is called a Ballard shell. A Ballard shell is a plated copper shell about 0.004 inches (0.10 mm.) thick which is removably clad on the outside of a gravure printing roll, over the base copper. A printing image is engraved into the Ballard shell and covered with a protective plated chrome layer. The roll clad with the Ballard shell is then used for printing in a rotogravure press. When the printing image is no longer needed, the printing roll can be recycled by manually stripping the used Ballard shell from the roll, then applying a new Ballard shell to the roll in its place.
Base copper preparation is only done once at the beginning of the Ballard shell process and has to be repeated only if the cylinder is damaged mechanically during transportation or correction. Since base copper preparation is not a regular task in the Ballard shell process, speed is not a concern. The quality of the base, however, is important, because the base serves as starting point for all of the subsequent plating operations. Once the base is prepared, the regular Ballard shell production can begin. The Ballard shell process is well known in the art.
The Ballard shell is a very simple process technology when the correct procedures are followed, yet the quality of its results is rather sensitive to changes in process variables and to changes in the quality of the copper base.
The Ballard shell was developed by Ernest G. Ballard in the 1920's and is therefore one of the oldest process technologies in gravure cylinder making. In all operations where cylinders frequently receive new engravings the Ballard shell has considerable advantages, because it eliminates machining time. The Ballard shell gained widespread popularity in both publication and packaging printing. With the technology available then, however, the real problems of the Ballard shell process could not be solved.
Today the pendulum is swinging back in favor of the Ballard shell. In Europe, the Ballard shell is presently seeing an enormous revival both in publication and in packaging printing. In Japan all of the major publication and packaging printers never switched away from the Ballard shell, because with their high production volumes base copper technologies were not cost efficient. In the U.S. the Ballard shell has many supporters that never stopped using it and is finding new ones again.
The Ballard shell process has several clear advantages. It is the process that requires the least investment in equipment and the least space (a preparation sink, a copper plating tank, and a polisher are enough to start). The meticulous washing of cylinders is of no concern for the Ballard shell process. Since the chrome layer is stripped together with the copper layer, ink rests do not affect the quality of the process. Use of the Ballard shell process eliminates the necessity of dechroming every cylinder and therefore reduces the need to treat effluent containing chrome in the waste water treatment plant. Furthermore, with the exception of the manual stripping work, the Ballard shell process is very easy to automate, thus eliminating all labor besides stripping and improving capacity and turnaround time. Finally, the Ballard shell process uses thin copper layers which keep copper plating times low.
A principal disadvantage of the Ballard shell process is that manual work cannot be totally eliminated, which poses a problem if the work flow of a whole plating department is to be fully automated.
A Ballard shell is stripped from a printing roll by opening the side or inner periphery of the Ballard shell that is plated over the side of the cylinder with a putty knife or a similar sharp object. After the side of the shell is opened, two stripes of copper are pulled across the cylinder like a zipper, and then the whole shell falls off the roll. The hardest part of stripping a Ballard shell is getting the first piece loosened on the cylinder side. The shell can be opened easily with a putty knife only if its inner periphery terminates abruptly at an edge which is easily engaged by a putty knife or a similar tool.
A first technological challenge of the Ballard shell is to plate an engraving copper layer that does not adhere to the base, and yet is fixed firmly on the roll so it will not come off prematurely in the press. A second challenge is to plate a Ballard shell that is soft and malleable enough so it can be stripped off, and yet is hard enough to be engraved electromechanically.
The preparation for plating a Ballard shell is a regular degreasing process with the additional process of applying the separation layer. The separation layer is either manually poured over the cylinder or automatically sprayed on. Separating solutions can be based on mercury, nickel, silver or protein.
The Ballard shell of the gravure cylinder is etched with ferrous chloride or electromechanically engraved to provide the image to be printed. After that, the image carrier is almost always covered with a thin electroplated layer of chrome (about 6 microns or 0.00023 inches thick) which protects the engraving. The chrome layer is added because the copper alone would not withstand the friction of the doctor blade for the long printing runs normally encountered.
The type of chrome that is used for rotogravure has a high hardness of around 1100 Vickers. This compares with a copper hardness of around 200 Vickers (hard copper for electromechanical engraving). Chrome also has a low coefficient of friction, and the ink carried by the chrome surface also serves to lubricate the doctor blade so that little abrasion is produced. A chrome plated engraved copper cylinder can run for millions of revolutions without wear and without changing the cell shape at all.
The plating tanks for electroplating a gravure cylinder with copper and chrome consist of anodes and an electrolyte trough that is chemically and electrolytically resistant to the electrolyte. The cylinder is the cathode (except when the current is briefly reversed in the copper plating bath). The cylinder and the anodes are connected to a rectifier that supplies the necessary DC current for plating. Both the cylinder surface and the anodes are immersed in a bath of the electrolyte carried in the trough. The plating tank has to be designed to control the process in a way that the desired reactions take place and the undesirable reactions are suppressed.
The immersion factor describes what percentage of the cylinder surface is immersed in the electrolyte at any point in time and is therefore available as an active surface which can receive plating. The higher the immersion factor, the faster the potential speed of the tank, because more current can be conducted through the large surface. A higher immersion factor normally also requires a larger anode and has an impact on plating tank design because of sealing requirements.
Current density is the amount of current flowing divided by the active area. The higher the current density for a given immersion factor, the faster the plating speed. The distance between the nearest anode and a point of the cylinder has a strong impact on the resistance at that point. The smaller the distance, the smaller the resistance at that point. Most modern copper and chrome plating tanks run with small anode/cathode distances, for example, from about one to two inches (25-51 mm.).
Contaminants of the electrolyte will be built into the copper or chrome surfaces along with the intended ions. They change the structure and characteristics of the plate, mostly in undesirable ways. Once attached to the cathode surface, they change the electric field in that area and lead to growth of "pimples" or "comets" that create problems in all further operations.
In particular, iron can enter the electrolyte when a ferrous surface of the cylinder is exposed to the plating bath, particularly if the current in the plating tank is reversed briefly just before plating is commenced, which is commonly done.
In every plating tank the cylinder has to be supported, driven, cathodically contacted and sometimes sealed. These tasks are carried out by the adapter system, which consists of the actual adapter for the cylinder and the clamping system in the tank. Adapters come in wide varieties, ranging from large screw-on current transfer adapters to small slide-on bushings to completely adapterless systems. Adapterless tanks are typically more expensive and need more maintenance than simple tanks, but they can save large amounts of labor. The decision for a specific adapter system has to be made in conjunction with the decision for the overall level of automation in the whole line.
All modern copper and chrome plating tanks have a horizontal design with adapter systems. They come with immersion factors ranging from under-shaft to full immersion.
According to one common method of forming a Ballard shell, the shell is plated using an under-shaft immersion tank which runs without seals. The cylinder is disposed horizontally and supported by its shaft in the tank so that the outer surface of the cylinder is immersed in the plating electrolyte to a depth not quite great enough to allow the electrolyte surface to touch the cylinder shaft. The cylinder is rotated during the plating process by turning its shaft to evenly plate the entire circumference of its outer surface. Since the sides of the cylinder are immersed in and not protected from the electrolyte, the wetted surface of each side is also plated in an annular pattern which extends radially inward to an inner periphery representing the depth of immersion of the cylinder side.
When the copper Ballard shell is plated on the cylinder in this manner, the thickness of the copper shell tapers off gradually between the outer edge and the inner periphery of the side of the cylinder. This characteristic taper occurs because the current density at a given point of the side, and thus the thickness of the shell at that point, is inversely proportional to the distance between the point and the anode. The inner periphery of the Ballard shell thus does not terminate abruptly at a sharp, thick edge, but tapers down to a feathered edge. In this case the shell is hard to open and the cylinder sides are liable to be scratched when a worker struggles to open the shell.
The inner periphery of the shell can be improved by painting the cylinder sides with acid resistant lacquer before the shell is plated. The lacquer is painted up to a distance of 10 mm to 25 mm below the edge of the cylinder. The copper now plates down to where the lacquer starts. The lacquer, however, does not have a specific thickness, so the copper cannot plate a ledge to it. A steep tapered ending is formed by the plating process. This steep tapered ending is much easier to open than when nothing is done, but a better ending would be far easier to open. In addition applying and removing the lacquer is labor intensive work.
In all plating tanks with higher immersion factors (up to 90%) the cylinder shafts are covered with polypropylene (or similar material) tubes with rubber sealing gaskets pressed against the cylinder sides. These sealing tubes are either put on manually or are built into the tank and move in automatically. Ideally, a different diameter tube is used for different cylinder diameters. The rubber gasket defines how far the copper plates down the side. The seal also acts somewhat as a current deflector, so the edge for the shell is tapered. It is much better to open the shell if sealing gaskets are used than if nothing is done, but still the opening operation is not optimal.
In all plating lines that are fully automated, the adapterless tanks with automatic sealing designs have a problem. The sealing diameter of the automatic seal necessarily corresponds to the smallest diameter of any cylinder that is processed in the line. This means that all other cylinders basically have the same problems as if nothing was done to create an edge.